In the field of magnetic recording media having a magnetic layer consisting mainly of ferromagnetic particles and binder resins, there is a growing demand for such media capable of higher-density recording. Smoothing the surfaces of the magnetic layers is one of the known measures to cope with such demand.
However, enhanced surface smoothness of the magnetic layer causes the coefficient of friction between the magnetic layer of the magnetic recording medium during running and the recording or reproducing device's parts in contact with the magnetic layer to be so heightened that the magnetic layer has been apt to suffer damage or to peel off in a relatively short period of use.
The magnetic layers of floppy disks, in particular, are often exposed to severe atmospheres as in durability tests. In the case where floppy disks are used under such conditions, the magnetic layers are occasionally apt to shed ferromagnetic particles, which can cause clogging of magnetic heads.
Further, as an effective means for obtaining magnetic recording media that can record information at high densities, ferromagnetic metal particles have come into use as ferromagnetic particles. The use of ferromagnetic metal particles in a floppy disk, however, has been disadvantageous in that due to the rubbing contact of the magnetic layer with the nonwoven fabric (liner) attached to the inside of the floppy disk jacket or the inside of the cartridge, the magnetic layer is prone to suffer damage in short periods of time, in turn potentially causing dropouts. This problem, which did not occur with conventional floppy disks in which the ferromagnetic particles were iron oxide particles, has often been encountered, particularly when the floppy disks are used under environmental conditions of relatively high temperatures such as 50.degree. to 60.degree. C.
For improving the running durability of magnetic layers, it has been proposed to incorporate an abrasive agent (hard particles), such as corundum, silicon carbide, or chromium oxide, in magnetic layers. However, this technique is disadvantageous in that in order to fully produce the effect of improving the running durability of magnetic layers, the abrasive agents should be incorporated into the magnetic layers in considerably large amounts.
Further, the incorporation of an abrasive agent has been found insufficient for protecting the magnetic layer from being damaged by the liner.
In addition, the magnetic layers containing large proportions of abrasive agents cause excessive magnetic head wear, etc. Furthermore, the incorporation of abrasive agents has been undesirable in that it is contrary to the approach of improving electromagnetic characteristics by smoothing magnetic layers.
It has also been proposed to incorporate in magnetic layers a fatty acid or an ester of a fatty acid with an aliphatic alcohol, as a lubricant, so as to minimize coefficient of friction. For example, U.S. Pat. No. 3,833,412 proposes to incorporate a monobasic fatty acid having 4 or more carbon atoms, such as caprylic acid, lauric acid, or oleic acid, as a lubricant; U.S. Pat. No. 3,274,111 proposes to incorporate as a lubricant a fatty acid ester which is an ester of a monobasic fatty acid having 12 to 16 carbon atoms with an aliphatic alcohol having 3 to 12 carbon atoms, examples of such ester including butyl laurylate, lauryl palmitate, and butyl myristate; and U.S. Pat. No. 4,303,738 proposes to incorporate tridecyl stearate as a lubricant.
JP-A-55-157131 proposes to incorporate as a lubricant an ester of a higher fatty acid with a branched higher alcohol, and also JP-A-59-186130 proposes to incorporate isocetyl stearate as a lubricant. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
However, the incorporation of such lubricants adversely affects the strength of the magnetic layers, although effective in lowering the coefficient of friction. Thus, the incorporation of lubricants has been found insufficient for preventing damage of the magnetic layer caused by contact with the liner.
Moreover, use of cellulosic resins, as binder resins, having high modulus of elasticity and excellent mechanical strength, so as to improve the mechanical strength of magnetic layers is disclosed in, for example, JP-A-56-13519, JP-A-62-239316, JP-A-58-70424, JP-A-58-70425, JP-A-56-74833, JP-A-60-133527, JP-A-62-34326, JP-A-62-26628, JP-A-59-79428, JP-A-57-135439, JP-A-56-74832, and JP-A-59-188827.
Binder resins based on such conventional cellulosic resins including nitrocellulose, however, are insufficient in the ability to disperse ferromagnetic particles, so that not only do the resulting magnetic layers not have good surface quality, but the packing density of ferromagnetic particles cannot be sufficiently increased. Consequently, the use of conventional cellulosic resins has been ineffective in providing magnetic recording media having excellent electromagnetic characteristics.
As another method for obtaining mechanically tough magnetic layers, use of a polyurethane resin, as a binder resin, obtained from a polyester polyol synthesized from an alicyclic or aromatic dihydric alcohol and an alicyclic or aromatic dicarboxylic acid is disclosed in, for example, JP-A-62-246310 and JP-B-58-8053; and, incorporation of such polyurethane resin is advantageous in heightening modulus of elasticity and mechanical strength. (The term "JP-B" as used herein means an "examined Japanese patent publication".)
However, binder resins employing the above polyurethane resin have been insufficient in their ability to disperse ferromagnetic metal particles and also in preventing the resulting magnetic layers from damage caused by contact with liners.